Wafer transfer systems are used to provide an automated transfer of semiconductor wafers from one position to another position. For example, the wafers contained in a cassette may be moved individually to a processing chamber for depositing and patterning layers of material for forming integrated circuit chips. Robotic handling devices are preferred, since human handling is more likely to cause contamination.
Conventional cassettes include slots that support the wafers in a vertical orientation. An automated wafer transfer system may then be used to move semiconductor wafers between a cassette and a processing chamber. U.S. Pat. No. 4,449,885 to Hurtel et al. describes an automated system that includes a cassette conveyor assembly for properly positioning a cassette of vertical wafers relative to an elevator blade which vertically raises and lowers individual wafers to and from a processing chamber. The elevator blade passes upwardly through the cassette to contact a lower edge of a wafer. The wafer is then raised to a position in which the elevator blade transfers the wafer to a vacuum chuck. Following the transfer, the vacuum chuck is retracted to position the wafer within the chamber. When the processing of the wafer has been completed, the vacuum chuck returns the wafer to a position for transfer to the elevator blade. The wafer is lowered to its original position within the cassette, and the cassette conveyor moves the cassette to a position aligned for pickup of the next wafer by the elevator blade. Transfer systems of this type require a precise positioning of the wafers within the cassette. The movement provided by the cassette conveyor between each lift operation must be compatible with the distance between the wafers within the cassette.
Another type of wafer storage device is referred to as a Standardized Mechanical InterFace (SMIF) pod. A SMIF pod is described in U.S. Pat. No. 5,653,565 to Bonora et al. The wafers are held in a horizontal orientation, rather than a vertical orientation. The wafers are located within the wafer-carrying cassette that is stored and transported between processing stations in the sealed SMIF pod. A SMIF pod for housing a cassette adapted for 200 mm diameter wafers may have a base with a rectangular footprint having a length of approximately 280 mm and a width of approximately 260 mm. The height of the pod is selected to accommodate a particular cassette. The pod includes a cover that mates with a door to provide a sealed environment for the wafers within the cassette. When the wafers are to be transferred to a processing station, the pod is placed onto an access port of a transfer system such that the pod door is in contact with the access port. In Bonora et al., the pod door is at a lower surface of the SMIF pod, so that when an elevator lowers the pod door, the cassette and its wafers are lowered into a processing chamber.
As noted in the Bonora et al. patent, there are different-sized wafers that require cassettes having different dimensions. Wafers having a diameter of 300 mm are now being used. Therefore, equipment has been designed to accommodate 300 mm wafers. One type of 300 mm SMIF pod includes a base having a circular footprint, as compared to the rectangular footprint of the 200 mm SMIF pod. Bonora et al. provide an adaptor plate that achieves "backward compatibility," i.e., the ability to use 200 mm SMIF pods with access ports of wafer processing equipment configured to accept 300 mm SMIF pods.
Another type of wafer container is described in U.S. Pat. No. 5,711,427 to Nyseth. The Nyseth wafer container is used for 300 mm diameter semiconductor wafers. The container is sometimes referred to in the industry as a Front Opening Unified Pod (FOUP), since an access door is located on a side that is perpendicular to horizontally stored wafers. Automated transfer systems for use with FOUPs have been designed. The transfer systems are significantly different than systems used with other types of containers, since the door is vertically mounted and the wafers are horizontally stored. Moreover, the center-to-center distance (i.e., pitch) of the wafers within a FOUP is greater than the center-to-center distance between wafers within a container of a different standard. For example, the pitch of 300 mm wafers within a FOUP is approximately 10 mm, while the pitch of wafers within a standard cassette of 200 mm wafers may be approximately 6.35 mm. The difference in the position of the door and the difference in the pitch render it difficult for FOUP-specific wafer transfer systems to be used to process 200 mm wafers. That is, the position of the door and the wafer pitch inhibit backward compatibility.
What is needed is a wafer-storage device which is convertible with respect to storing different-sized semiconductor wafers for presentation to automated transfer equipment specifically designed for handling one of the wafer sizes.